This is the first report on analyzing the chemical state of Li-ion battery electrodes at different states of charge by using a wavelength-dispersive spectrometer, which has a two-order improved energy resolution in the soft X-ray energy region compared with that of a conventional energy-dispersive X-ray analyzer. Electrodes containing LiMn1.5Ni0.5O4 were charged to prepare Li0.5Mn1.5Ni0.5O4 and λ-Mn0.75Ni0.25O2. The soft X-ray emission spectra obtained from those materials show that the O-K emission signal was drastically decreased throughout the charging process. This suggests that O-2p electron contributed to the electrochemical oxidation. The density of states and Bader charge evaluated from ab initio calculation support this result.
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